Krugersdorp, South Africa
Krugersdorp, South Africa

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Combrinck L.,Space Geodesy Programme | Combrinck L.,University of Pretoria
South African Journal of Geology | Year: 2011

Parameterised post Newtonian (PPN) parameters γ and β (which should equal unity if general relativity is valid) are estimated through the analyses of four years of LAGEOS 1 and LAGEOS 2 data. To reduce the chance of possible imprinting of general relativistic effects which could be contained in the Earth gravity model used, several low order spherical harmonic coefficients are solved in the orbit estimation and propagation process. After convergence is reached, the solved PPN parametersand γ and β are kept fixed at their solved values and scaling coefficients are introduced to explore the possibility of open parameter space in the accelerations due to the Schwarzschild field, frame dragging and de Sitter (geodesic) precession. It is found that some parameter space is available in the radial acceleration component, which could be taken up by including post-post-Newtonian components of general relativity, or it could be due to some other radial acceleration component that is not adequately modelled. The inclusion of scale coefficients in the least-squares process proves to be a valuable tool in evaluating the solutions for γ and β. A comparison with results obtained for the PPN parameters by other authors utilising different techniques such as VLBI and radar measurements to planets and probes are made. © 2011 December Geological Society of South Africa.


Botai M.C.,Space Geodesy Programme | Botai M.C.,University of Pretoria | Combrinck L.,Space Geodesy Programme | Combrinck L.,University of Pretoria
South African Journal of Geology | Year: 2011

Continuous tracking of geodetic satellites using the Satellite Laser Ranging technique has provided unprecedented opportunity in long- to medium-wavelength gravity field modelling. Numerous gravity field models have been derived from such observations and have been made freely available to the science community for research purpose. The accuracy of most of the latest gravity field models in terms of precise orbit determination is currently at cm level. Improvement in the Earth gravity field modelling is anticipated as quantitative and qualitative data (in particular from low earth orbit satellites) become available in the future. Such expectations require that the accuracy and precision of existing gravity field models be assessed. The validation of gravity field models in terms of satellite orbit determination is often based on the difference between the observed and computed range. The resulting range residuals are considered an important index when determining the accuracy of the gravity models and hence the satellite orbits. In this contribution we investigate the general improvement in gravity field modelling over a period of 15 years. The orbit accuracy of twelve gravity field models (both satellite-only and combined models) were assessed by analysing seven months of data from LAGEOS 1 and 2 using HartRAO analysis software. Results show that the gravity field models developed over the years have improved by at least a factor of 2 since 1990, considering improvement in O-C residuals. © 2011 December Geological Society of South Africa.

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